Process for removing contaminant

ABSTRACT

Contaminant is removed from the interior of the holes in the vicinity of preselected locations by etching in a gaseous plasma wherein the sheath voltage is controlled in order to direct ions of the plasma to contact the interior of the holes in the vicinity of the preselected locations.

DESCRIPTION TECHNICAL FIELD

The present invention is concerned with removing contaminant from holesand particularly from holes provided in circuit boards and circuitcards. The present invention is especially concerned with removingcontaminant by subjecting the article to a gaseous plasma. The presentinvention provides for selectively removing contaminant from theinterior of the holes within the vicinity of preselected locationswithin the holes without over-etching the outer edges of the hole.

BACKGROUND ART

In the manufacture of printed circuit cards and boards, a dielectricsheet material is employed as a substrate. A conductive circuit patternis provided on one or both of the major surfaces of the substrate.

Multilayer cards and boards are typically constructed of a number oflayers of copper conductor patterns separated by layers of dielectricplastic. These structures can be produced by laminating techniques.

In order to provide electrical connection between layers, holes referredto as "through-holes" are provided in the circuit boards and cards andsuch are subsequently plated with an electrically conductive coatingsuch as copper.

However, in the process for producing the holes, invariably some amountof contaminant is formed within and around the edges of the holes.

For instance, when drilling a dielectric substrate such as obtained froman epoxy material, what is commonly referred to as "epoxy smear" due tothe drilling results within the hole and around the edges of the hole.Accordingly, prior to electrically plating the holes, the contaminantmust be removed to as great an extent as possible. Various treatmentshave been suggested for removing such contaminant including subjectingthe substrate to a gaseous plasma. However, it has been observed thatwhile many gaseous plasma systems are effective for removing thecontaminant within the vicinity of the edges of the holes that such aremuch less effective in removing contaminant from the interior of theholes and particularly within the vicinity of the interplanes of theholes.

The interplanes of the holes represent those locations within the holeswhere the copper conductor layers exist.

One troublesome problem with plasma etching to remove contaminant fromholes is that if the etching process is continued for sufficient time inorder to remove substantial contaminant from the interior of the holes,then the outer edges of the holes due to the additional etching timebecome over-etched. These problems are particularly pronounced whendealing with holes having a relatively high aspect ratio. The aspectratio is the depth of the hole relative to its diameter.

The above-discussed problem, as well as one particular technique forovercoming the problem, is disclosed in U.S. Pat. No. 4,654,115 toEgitto, et al., disclosure of which is incorporated herein by reference.

SUMMARY OF INVENTION

The present invention is concerned with a process for removingcontaminant from holes.

In particular, the present invention is concerned with a process forremoving contaminant selectively from the interior of holes in thevicinity of preselected locations within the holes by etching in agaseous plasma.

It has been found, in accordance with the present invention, that theions of the plasma can be selectively directed to contact the interiorof the holes in the vicinity of preselected or predetermined locationsby selecting or controlling the value of the sheath voltage. The sheathvoltage is that voltage which exists outside of the hole between thesubstrate and the plasma.

SUMMARY OF DRAWING

The FIGURE is a schematic diagram of a through-hole and proximity to aplasma.

BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION

The process of the present invention is concerned with removingcontaminant from holes and is especially concerned with removingcontaminant from holes provided in dielectric substrates; particularlythose employed for use in electronic circuit boards and circuit cards.

In particular, the present invention is concerned with removingcontaminant from preselected locations within the interior of the holessuch as in the vicinity of the interplanes of the holes. It isespecially important to remove contaminant from the vicinity of theinterplanes within the holes in order to assure adequate electricalconnections with the conductor patterns located in the various layers ofthe cards or boards.

Holes in various dielectric substrates including substrates fromthermoplastic and thermosetting resins may be treated pursuant to thepresent invention.

Typical thermosetting polymeric materials include epoxy, phenolic-basedmaterials, polyimides, and polyamides. The dielectric material may bemolded articles of the polymers containing fillers and/or reinforcingagents such as glass-filled epoxy or phenolic-based materials. Examplesof some phenolic-type materials include copolymers of phenol,resorcinol, and cresol. Examples of some suitable thermoplasticpolymeric materials include polyolefins such as polypropylene,polysulfones, polycarbonates, nitrile rubbers, and ABS polymers.

Typical epoxy resins include the bisphenol A type resins obtained frombisphenol A and epichlorohydrin, resinous materials obtained by theepoxidation of novolak resins produced from a phenolic material such asphenol and an aldehyde such as formaldehyde with epichlorohydrin,polyfunctional epoxy resins such as tetraglycidyldiaminodiphenylmethane, and alicyclic epoxy resins such asbis(3,4-epoxy-6-methyl-cyclohexamethyl) adipate. The most preferredepoxy employed is of the bisphenol A type.

Also, the resinous epoxy compositions can contain accelerating agentsand curing agents, as well-known in the art. Examples of suitable curingagents include polyamines, primary, secondary, and tertiary amines,polyamides, polysulfones, urea-phenol-formaldehyde, and acids oranhydrides thereof. In addition, suitable curing agents include Lewisacid catalysts such as BF₃ and complexes thereof.

The dielectric substrate usually contains fillers and/or reinforcingfibers such as glass fibers. Such compositions containing fibers areusually prepared by impregnating the fibers with the epoxy composition.The amount of the epoxy composition when combined with the fibers isusually about 30% to about 70% by weight and preferably about 55% toabout 65% by weight of the total of the solids content of the epoxycomposition and the fiberglass.

After combining with the reinforcing fibers, the composition is cured tothe B-stage and molded to the desired shape, such as a sheet commonlyreferred to as a prepreg. When sheets are employed, the thickness isusually about 1.5 mils to about 8 mils and preferably about 2 mils toabout 3 mils. The curing to the B-stage is generally achieved by usingtemperatures of about 80° C. to about 110° C. and for times of about 3minutes to about 10 minutes.

After the prepreg is formed, a sheet of copper or other conductivematerial can then be laminated to one or more layers of prepreg usinglaminating conditions such as about 50 psi to about 800 psi, and moreusually about 200 psi to about 500 psi, at about 150° C. to about 200°C. for about 30 minutes to about 5 hours. Then a circuit can be etchedto the conductive layer using techniques well-known to form circuitboards. Multilayer boards and cards are formed by laminating togetherthe desired number of such boards.

The through-holes are provided in the dielectric substrate and must besubsequently provided with an electrical conductor such as copper inorder to provide electrical connection between circuits on opposingsurfaces of the dielectric substrate. These through-holes can beprovided by mechanical operations such as drilling and punching or byprocessing, such as by use of a laser. However, the formation of theholes results in contaminant present in the interior of the holes andaround the vicinity of the edges of the hole which is generally referredto as "smear". A large portion of this contaminant is from thedielectric material in the substrate; for instance, when the dielectricis an epoxy, the contaminant is predominantly epoxy smear.

It is essential for reliable plating of the through-holes to remove asmuch of the contaminant as possible. Removal of contaminant selectivelyin the interior of the holes within the vicinity of preselectedlocations within the holes is accomplished in accordance with thepresent invention. The contaminant is removed by etching in a gaseousplasma.

Objects that are placed into a gaseous plasma acquire a negative biaswith respect to the plasma. This bias causes the object to be bombardedwith positive ions as discussed, for example, in Chapman, Glow DischargeProcesses, Wiley. In order to provide for the ions to contactpreselected locations within the interior of the holes, according to thepresent invention, the sheath voltage is properly controlled so thatthere exists a curved path for the ions directed to the preselectedlocations. This curved path is related to the radial electric fieldbetween the axis of the holes and the preselected locations within theholes, such as the interplanes. The electric field is represented by thefollowing known equation: ##EQU1## wherein: Q is the total charge ofions in the hole

ε_(o) is the permitivity of free space

R is the radius of the hole

L is the length of the hole

This electric field E causes the ions to curve towards the walls withinthe hole.

In accordance with the present invention the sheath voltage iscontrolled to direct the ions of the plasma to contact the predeterminedlocations within the holes.

The speed of the ions (axial velocity) must be such that the ions do notovershoot or undershoot the predetermined locations within the holes.For instance, if the axial velocity of the ions is too large, then theions will pass through the holes without contacting the walls at thepredetermined locations. If the axial velocity of the ions is too slow,then the ions will contact the interior of the hole in the vicinity ofthe entrance of the holes.

The sheath voltage can be manipulated by several different techniques.Some methods of controlling the sheath voltage are controlling the RFpower to the plasma, the frequency of the RF power to the plasma,providing a separate independent electrode in the system, and placing anappropriate voltage in it, creating a plasma difference between appliedRF power to the plasma and RF power directly applied to the board orcard, and providing electrodes into the board or card to generate powerwithin preselected locations within the hole.

Whichever method is selected, the particular sheath voltages can beselected by persons skilled in the art without undue experimentation forthe particular conditions involved once such persons are aware of thepresent disclosure and invention. For instance, see Keller, et al.,"Electrical Properties of RF Sputtering Systems", IBM Journal ofResearch and Development, Vol. 23, Number 1, January 1979, disclosure ofwhich is incorporated herein by reference, for a discussion of sheathvoltage and manipulation of sheath voltage.

There are three methods for controlling the sheath voltage discussed byKeller, et al. These methods are referred to as:

1. Tunnel substrate

2. Driven

3. CARE.

In particular, with the substrate drive voltage V_(d) equal to zero, thesystem is a tuned substrate system; with the impedance Z_(es) equal tozero, it is a driven system; with both equal to zero, it is a controlledarea ratio electrode (CARE) system.

In the tuned substrate system a variable inductor and a blockingcapacitor are placed in series between the substrate and ground (straycapacitance to ground of the order of 100 pF or more.) The tuningnetwork is usually designed to adjust over a range including both seriesresonance with the blocking capacitor and parallel resonance with thestray capacitance.

In a driven substrate system some of the rf power from the generator isused to drive the substrate holder at a given rf voltage and phase. Twosynchronized generators may also be used for driving the target andsubstrate.

In a CARE system the plasma voltage corresponds to the ion bombardmentpotential at the substrate. CARE stands for Controlled Area Ratio ofElectrodes. Equation 35 given by Koenig and Maissel, IBM JournalResearch Development, 14, 168(1970), below shows that the plasma voltageV_(p) depends on the area ratio of electrodes A_(t) /A_(w) and A_(s)/A_(t). ##EQU2##

V_(t) refers to the target voltage and V_(d) refers to the substratedrive voltage. Thus, the sheath voltage which is proportioned to theplasma voltage; can be adjusted by adjusting the area of the electrodesin contact with the plasma.

By manipulating the sheath voltage, a plurality of locations within theholes can be contacted by the plasma. For instance, this can beaccomplished by scanning the voltage from one preselected location tothe next preselected location such as by using a variable DC voltagesource and scanning or changing it from one value for the firstpreselected location to the next value for the next preselectedlocation.

To facilitate understanding of the present invention, reference ishereby made to the FIGURE wherein numeral 1 represents the substratehaving holes (2) therein and interplanes (3) and (4). The substrate (1)is shown located on the electrode (5) of the plasma generating apparatuswhich, in turn, is connected to variable DC voltage (6). Upon creationof the plasma, a sheath voltage (7) is generated between the substrateand plasma as shown by arrows (8,8). By properly varying the sheathvoltage, such as by varying the DC voltage, the ions of the plasma canbe made to first contact interplane (3) and then interplane (4).

The present invention is especially advantageous for removingcontaminant from the interior of relatively high aspect ratio holes,such as at least about 3:1 and generally at least about 9:1.

The sheath voltage is usually about 15 volts to about 750 volts and moreusually about 50 volts to about 250 volts.

The gaseous plasma is formed from an oxygen-containing gas and afluorinated compound. The relative amounts of the oxygen-containing gasand the fluorinated compound are selected so as to remove contaminantfrom the interior of the holes in the vicinity of the preselectedlocations.

Preferred oxygen-containing gas employed is oxygen per se. Suitablefluorinated compounds include CF₄, C₂ F₆, CFCl₃, CF₃ Cl, SF₆, CCl₂ F₂,and NF₃ with the most preferred fluorinated compound being CF₄.

The relative amounts of the oxygen-containing gas and the fluorinatedcompound will depend upon the specific fluorinated compound employed andthe particular dielectric substrate material. However, the amounts canbe readily determined without undue experimentation by merelydetermining the maximum etch rates as a function of different gas-feedcompositions. Once this is determined for a particular set ofconditions, the ranges can then be employed as discussed hereinbelow.

For instance, with CF₄ as the fluorinated compound, mole ratios of O₂ toCF₄ of 60 to 90:40 to 10 are suitable for an epoxy dielectric substratefor this etching step. A typical ratio is about 70:30 of oxygen/CF₄. Onthe other hand, in the same system employing SF₆ as the fluorinatedcompound, the ratio of oxygen/SF₆ is about 95 to 80/5 to 20.

This etching step of the process is usually completed within about 5minutes to about 45 minutes and more usually about 10-25 minutes and isgenerally carried out from about room temperature to less than thedegradation temperature of the dielectric substrate, and more usuallyabout 50° C. to about 130° C. The gas residence time is usually about 1second to about 5 minutes, and more usually about 1 second to about 1minute.

The pressure employed is generally about 100 millitorr to about 500millitorr. Typical power levels are about 0.05 watts to about 2 wattsper square centimeter of one major surface of the dielectric substratebeing treated.

Plasma reactors suitable for carrying out the process of the presentinvention are commercially available and need not be discussed herein inany detail. Typical commercially available plasma reactors suitable forcarrying out the present invention are Branson IPC-Parallel PlateReactor Model 74-15; in-line plasma system available from Koksai,Applied Plasma System's plasma reactor; and Technics' plasma reactor.

The process of the present invention provides for selective etching ofcontaminant from the interior of the holes with a minimum of unnecessaryetchback or overetching away from the preselected locations within thehole. Furthermore, the present invention makes it possible to increasethe speed of the overall process, thereby providing advantages of acommercial nature.

Having thus described out invention, what we claim as new and desire tosecure by letters patent is:
 1. A process for removing contaminant fromholes located in a substrate and having an entrance, outer edges andinterior walls which comprises etching said holes having contaminantpresent on the interior walls of the holes in a gaseous plasma toselectively remove contaminant from the interior of the holes inpreselected locations within said holes whereby a sheath voltage isgenerated between said substrate and said plasma by controlling thevalue of the sheath voltage to direct ions of the plasma in a curvedpath to contact the interior of said holes in the vicinity of saidpreselected locations in order to selectively remote contaminanttherefrom without overetching the outer edges of the holes.
 2. Theprocess of claim 1 wherein said preselected locations are interplanes.3. The process of claim 1 wherein said sheath voltage is varied from afirst value to a second value to cause ions of the plasma to contactfirst preselected locations within said holes and then to contact secondpreselected locations within said holes.
 4. The process of claim 1wherein the aspect ratio of said holes is at least about 3:1.
 5. Theprocess of claim 1 wherein the aspect ratio of said holes is at leastabout 9:1.
 6. The process of claim 1 wherein the sheath voltage iscontrolled so that said ions of the plasma do not contact the interiorof the holes in the vicinity of the entrance of the holes.
 7. Theprocess of claim 1 wherein said gaseous plasma is formed from oxygen anda fluorinated compound.
 8. The process of claim 7 wherein thefluorinated compound is selected from the group of CF₄, C₂ F₆, CFCl₃,CF₃ Cl, SF₆, NF₃, and CCl₂ F₂.
 9. The process of claim 7 wherein saidfluorinated compound is CF₄.
 10. The process of claim 1 wherein saidholes are located in dielectric substrate.
 11. The process of claim 10wherein said dielectric substrate is an epoxy substrate.